Good game but i feel like the controls need improvement.
It's a very nice game, plays smoothly and it's a fun concept/theme, but even if you shoot prime numbers, you shouldn't need to literally press them. As an exmaple:
The game is in portrait mode which immediately reminds me of a mobile game, right? But on a mobile you yould tap on buttons that are most likely next to eachother. The same applies to PC, you can just use one finger and press one at a time, but if you are used to a keyboard you won't do that and 2, 3, 5 and 7 feels unnatural and also a bit uncomfortable.
This is a just a suggestion of course, and it's based on personal preference, but please consider giving it a try using for example the hotkeys 1234 or qwer as input keys.
Cheers, i hope this helps :)